When the current version was released, I was notified via PdN when I first opened it up. It said something like "There is a new version of Paint.net available, would you like to upgrade?" or something along the lines of that. So yeah, and if that doesn't work, go to Utilities => Check for updates (Make sure you go to the options tab and have Automatically check for new versions of Paint.net checked (You can check Also check for pre-release (beta) versions check if you desire))
Edit - [sarcasm] Darn you pyrochild! [/sarcasm]